Materials such as optical glass can be formed as convergent or divergent lenses. One problem inherent in optical lenses is that when two light beams having significantly different wavelengths pass through a lens material, they will become focused at different points, due to wavelength dispersion. Mathematically, dispersion is defined as the rate of change of the index of refraction (n), with respect to the wavelength (.lambda.) or D dn/d.lambda.. As different wavelengths are transmitted through the lens, the lens exhibits a different index of refraction for each wavelength. Thus, different wavelengths are refracted differently, and thereby focus at different points.
This focusing anomaly for greatly differing wavelengths can be undesirable in certain applications, such as laser surgery. Due to certain desirable properties of laser light outside the visible portion of the spectral range, physicians often perform laser surgery using laser light that is invisible to the human eye, e.g., wavelengths in the far infrared. Because this light cannot be seen by the surgeon, medical laser systems utilizing invisible light typically employ a low power "aiming" laser beam at a visible wavelength. The aiming beam is focused on a patient where an incision is to be made, and then a high power laser beam at an invisible wavelength is applied to make an incision at the point where the low powered beam is focused. Typical medical laser systems employ a helium neon laser, emitting a beam of approximately 0.63 microns wavelength which is not damaging to biotic material, for the low powered aiming beam and a carbon dioxide laser, emitting a beam of approximately 10.6 microns wavelength which vaporizes biotic material, for the high powered cutting beam. The two beams must focus at the same point to ensure the incision is made at the desired location. Even small differences in the location of the focal points of the beams can cause the tissue of a patient to be cut improperly.
Achromatic lens elements are commonly used to align the focal point of two different wavelengths passing through the same optical system. Such elements comprise two different materials which, together, correct for the focusing anomaly caused by dispersion. The different materials of the achromatic lens elements have substantially different indices of refraction and dispersion relationships such that they focus two beams of substantially different wavelengths at the same focal point. However, achromats have typically been used within a moderate range of wavelengths since it is difficult to find two materials which achromatically focus and transmit light over a large wavelength separation. The choices of materials are limited not only by achromatic compatibility, but by absorption properties. Typical optical glasses, for example, do not transmit well at wavelengths approaching 2.7 microns or more due to strong water absorption peaks in the vicinity of three microns. These problems have severely restricted the use of achromats in medical laser systems.
Accordingly, there is a need in the art for an achromatic optical element which will focus widely separated wavelengths, particularly where one of the wavelengths is in the visible portion of the optical spectrum and the other is in the far infrared.